[flang] [OpenMP] Name Resolution for OpenMP constructs (flang-compiler/f18#940)

[flang] [OpenMP] Name Resolution for OpenMP constructs (flang-compiler/f18#940)

This is an extended framework based on the previous work that addresses
the NR on OpenMP directives/clauses (b2ea520). In this change:
  * New `OmpVisitor` is created (ResolveNamesVisitor derives from it) to
    create necessary scopes for certain OpenMP constructs. This is along
    with the regular Fortran NR process.
  * Old `OmpVisitor` is adjusted and converted to a standalone visitor--
    `OmpAttributeVisitor`. This is used to walk through the OpenMP constructs
    and do the NR for variables on the OpenMP directives or data references
    within the OpenMP constructs. "Do the NR" here means that based on the NR
    results of the regular Fortran NR, fix the symbols of `Names` related
    to the OpenMP constructs. Note that there is an `OmpContext` in this
    visitor (similar to the one in `OmpStructureChecker`), this is necessary
    when dealing with the nested OpenMP constructs in the future.

Given an OpenMP code:
```
real*8 a, b
  a = 1.
  b = 2.
  !$omp parallel private(a)
  a = 3.
  b = 4.
  !$omp end parallel
  print *, a, b
end
```

w/o -fopenmp:
```
real*8 a, b
 !REF: /MainProgram1/a
 a = 1.
 !REF: /MainProgram1/b
 b = 2.
 !!!! OMP parallel
 !REF: /MainProgram1/a
 a = 3.
 !REF: /MainProgram1/b
 b = 4.
 !!!! OMP end parallel
 !REF: /MainProgram1/a
 !REF: /MainProgram1/b
 print *, a, b
end
```

w/ -fopenmp:
```
real*8 a, b
 !REF: /MainProgram1/a
 a = 1.
 !REF: /MainProgram1/b
 b = 2.
!$omp parallel  private(a)   <-- new Symbol for 'a' created
 !DEF: /MainProgram1/Block1/a (OmpPrivate) HostAssoc REAL(8)
 a = 3.   <-- fix the old symbol with new Symbol in parallel scope
 !REF: /MainProgram1/b
 b = 4.   <-- do nothing because by default it is shared in this scope
!$omp end parallel
 !REF: /MainProgram1/a
 !REF: /MainProgram1/b
 print *, a, b
end
```

Please note that this is a framework update, there are still many
things on the TODO list for finishing the NR for OpenMP (based on
the `OpenMP-semantics.md` design doc), which will be on top of this
framework.

Some TODO items:
- Create a generic function to go through all the rules for deciding
  `predetermined`, `explicitly determined`, and `implicitly determined`
  data-sharing attributes. (This is the next biggest part)
- Handle `Array Sections` and `Array or Structure Element`.
- Take association into consideration for example Pointer association,
  `ASSOCIATE` construct, and etc.
- Handle all the name resolution for directives/clauses that have
  `parser::Name`.

* N.B. Extend `AddSourceRange` to apply to current and parent scopes
  - motivated by a few cases that need to call `AddSourceRange`
    for current & parent scopes; the extension should be safe
  - global scope is not included

Original-commit: flang-compiler/f18@0c3c39d30e3f166a6a1303337c5fd7eead720fd0
Reviewed-on: https://github.com/flang-compiler/f18/pull/940